World Trade Center 7 was the third skyscraper destroyed on September 11, 2001. It was not hit by a plane. The picture on the left shows WTC 7 after the collapse of the Twin Towers, smoldering in the background.

The final report on its collapse, postponed several times, was published over 7 years after the event on November 20, 2008.

This steel-framed skyscraper, completed in 1987, was located 110 meters (350 feet) away from the closest of the Twin Towers ("WTC 1" on the map below). The building's tenants included the CIA, the Internal Revenue Service, several banks, the Mayor's Office of Emergency Management, and the U.S. Securities & Exchange Commission. A large number of records of ongoing investigations of Enron and other companies were destroyed with WTC 7.

No airplane hit WTC 7, but its south facade was damaged by debris ejected from the North Tower, which collapsed at 10.30 am. There is conflicting information about the amount of damage. For example, in the pictures shown in a preliminary official report the southwest corner is badly damaged, whereas in the photograph taken by Aman Zafar in the afternoon the same corner is intact – see my photo comparison. The building was reported on fire at 4.10 pm by CNN, although the fires seem to have started in the morning after the destruction of the North Tower. The fires, whose origin has not been determined, appeared on a number of floors, and the evacuated building collapsed at 5.20 pm.

A high-resolution video of the collapse is available here. The video below shows the totality and symmetry of the building's destruction.

The Federal Emergency Management Agency, FEMA, speculated that office fires caused the collapse of the building. It, however, acknowledged in its report in May 2002: "The specifics of the fires in WTC 7 and how they caused the building to collapse remain unknown at this time. [...] the best hypothesis has only a low probability of occurrence." Later in 2002, Larry Silverstein, the owner of WTC 7, gave in the America Rebuilds TV program the famous "pull it" statement that many have interpreted as meaning that the building was professionally demolished.

Did WTC 7 collapse as a result of office fires, or was it demolished with explosives? The answer can be sought by examining the way in which the building collapsed.

Collapse Speed

As one can observe from the videos of WTC 7's collapse, after the roofline began its smooth descent, the building fell to the ground in approximately 6.5 seconds. This is a phenomenally short time: a stone dropped from the top of the building would have reached the ground (covering a distance of 174 meters) in 5.95 seconds – if there were no air resistance! However, in principle the distance analyzed should be that from the top of the building to the top of the debris pile, not to the ground. As the exact height of the debris pile is not documented, it is more useful to examine the early stages of the collapse, during which the debris pile does not need to be taken into account.

According to the video analysis presented in the 9-11 Eyewitness documentary, starting from the state of rest, WTC 7 fell 100 meters in 4.5 seconds. This results in an acceleration of 9.8 m/s2, corresponding to a free fall.

To verify this, I examined the fall of a corner of the building in one collapse video using Blaze Media Pro video editing software. The corner fell 56 meters (=the distance between the Start and End lines in the animation below) in 3.47 seconds. This results in an acceleration of 9.3 m/s2, which corresponds to a very low resistance factor of the structural supports: only 5 percent of the force of gravity of the building's falling upper section.

I also measured an acceleration of 8.5 m/s2 for the middle part of the wide facade in the picture, which means that the force of resistance of the structural supports was 13 percent of the force of gravity of the falling upper section (see the calculation in more detail here).My measurements therefore support the conclusion presented in 9-11 Eyewitness that the skyscraper fell virtually unimpeded. The lack of structural resistance seems to be explainable only by the destruction of the structural supports through the use of explosives. WTC 7 dropped rather than collapsed. It came down as if only air had separated the roof of the building from the street below.

WTC 7 fell on average 7 floors per second (47 / 6.5). One second after the onset of the collapse, the speed of descent was almost 10 meters/second; after two seconds, almost 20 meters/second; and at the end, about 60 meters/second (over 200 kilometers/hour). According to the analysis of Frank Legge (Ph.D.), the rate of descent of WTC 7 closely matches the rate of gravitational free fall, which – combined with the uniformity of the descent throughout the breadth and length of the building – is irrefutable evidence of controlled demolition.

Heikki Kurttila, a Finnish Doctor of Engineering and accident researcher, has made detailed calculations about the collapse speed of WTC 7. He concludes that the short collapse time and low structural resistance "strongly suggest controlled demolition". Kurttila notes that an apple dropped from the height of WTC 7's roof would have taken about 0.5 seconds longer to reach the ground than it took the skyscraper to be completely destroyed.

Structural Features of the Collapse

A striking feature in the collapse of WTC 7 is symmetry. The collapse progressed evenly throughout the building, and the debris piled up almost completely within the foundations of the building (see the picture below).

The symmetry of WTC 7's descent means that all of its steel supports – 25 central and 58 peripheral columns – were destroyed almost simultaneously. Any asymmetry in the damage to structures would have led to asymmetrical collapse. By contrast, a symmetrical collapse without the controlled use of explosives would violate the principle of least resistance. Local office fires (typically dying out in about 20 minutes in any given location) and structural damage here and there could not have weakened all the central and peripheral support structures in a way that would have caused all of them to fail at the same moment. The simultaneity of the destruction of support structures throughout the building can, however, be explained by controlled demolition.

A controlled demolition is also suggested by the drop of the center of the skyscraper moments before the surrounding structures started to fall, as well as by the fact that the outer walls were pulled inwards. In a controlled demolition the collapse is caused by first destroying the weight-carrying core of a building, which "pulls" the exterior walls inwards ("implosion"). Although the lowest floor with fires was reportedly the sixth floor, the building seems to have undergone a traditional demolition, beginning from the bottom floor. An emergency worker who witnessed the collapse of WTC 7 was interviewed on 9/11. He described hearing what sounded like a "clap of thunder", followed by what looked like "a shockwave ripping through the building", with windows busting out, and "about a second later the bottom floor caved out and the [rest of] the building followed after that". The videos showing the collapse support his description.

At least one high-resolution video of the collapse of WTC 7 clearly shows one more characteristic of controlled demolition: streamers of dust emerging out of the building.

A Dutch demolition expert Danny Jowenko (right), who owns a demolition company and has been in the business for almost 30 years, concluded in September 2006 that WTC 7 "is controlled demolition. [...] A team of experts did this. This is professional work, without any doubt." A number of other demolition experts agree.

Hugo Bachmann, a Swiss professor emeritus for structural design and construction, said in Tages-Anzeiger: "In my opinion WTC 7 was with great probability brought down by controlled demolition done by experts". In addition, Jörg Schneider, another Swiss Professor emeritus for structural design and construction, interprets the existing videos as indicating that "WTC 7 was with great probability brought down by explosives".

Fire Endurance of Steel

Steel is very fire-resistant material. In tests conducted by Corus Construction in several countries, the fire endurance of steel-framed parking garages was examined by feeding fires with hydrocarbon fuel. Steel beams and pillars heated to a maximum of 360 degrees Celsius, and the breaking of steel was not even close. In Cardington fire endurance tests, modelled on conditions in real buildings, unprotected steel was subjected to temperatures of up to 1100 degrees Celsius (2012 F), but there was no collapse. Similarly, in the fire experiments contracted by the National Institute of Standards and Technology (NIST), testing the pancake collapse theory by modelling the fire exposure of WTC's floor assemblies, there was no collapse. Although NIST ignored the results in its final report, it acknowledged that the results "established that this type of assembly was capable of sustaining a large gravity load, without collapsing, for a substantial period of time relative to the duration of the fires in any given location on September 11th."

In the picture of WTC 7 to the right, the fires are limited to small areas, almost all windows are intact, and no red heat indicative of temperatures capable of softening steel is visible. The situation is largely the same in other photographs taken of the building in late afternoon. In some videos, such as this, a fair amount of smoke can be seen emanating from the southern facade (some of the smoke appears to be rising from WTC 5 and WTC 6 as well as from the remains of the twin towers), and several windows were broken on a few floors in the southern wall. In any case, WTC 5, which was badly damaged by the collapse of the North Tower next to it, burned much more powerfully. However, although this building had weaker support structures than WTC 7, it did not collapse into a debris pile, but remained standing – as did all the other highrises equally close to the Twin Towers, including the Verizon, Deutsche Bank, and U.S. Post Office buildings.

In December 2007, it was acknowledged in the advisory committee meeting of the NIST that the fires in WTC 7 were ordinary office fires and burned out in any given location in 20 minutes. In such a short time, the temperature of fire-protected steel members would have maxed out below 200 degrees Celsius (or ~400 degrees Fahrenheit). Such temperatures have no effect on construction steel.

Characteristics of the Debris

The debris of WTC 7 was extremely hot for weeks after the collapse of the building. Thermal imaging by NASA showed that the top of the debris pile had a temperature of 730 degrees Celsius five days after the collapse. Deeper, and immediately after the destruction, temperatures were probably considerably higher. Residual temperatures like this cannot be explained by office fires or by an ordinary, gravity-driven collapse. When the potential energy of a building experiencing an ordinary gravitational collapse turns into thermal energy, the result is only a few degrees' average increase in temperature.

According to several reports, molten metal (also suggested by this video footage) was found under the debris pile of WTC buildings. To melt structural steel, temperatures exceeding 1500 degrees Celsius are required. Such temperatures are never achieved in office fires. In addition to molten metal, partly evaporated steel beams were found in the debris of WTC 7. As professor Jonathan Barnett pointed out in a New York Times interview, the fires in the building could not have produced temperatures capable of evaporating steel. However, the use of explosives like thermite can produce temperatures (even 3000 degrees Celsius) that can melt and even evaporate steel.

FEMA's investigators were not allowed to work in the collapse zone itself. They were allowed to examine the debris of WTC skyscrapers only in landfill areas used as temporary storage for the steel debris before its recycling. By May 2002, when FEMA finished its preliminary report calling for further investigation, almost all the steel debris had been sold and shipped into the Far East. Only 236 pieces of steel had been retained for examination, of which a ridiculous total of 4 were from WTC 7. Even these may no longer exist.

As WTC 7 was evacuated over six hours before its destruction, there were no grounds for the rapid removal and recycling of the steel debris. Quite the contrary: as WTC 7 was one of the three greatest building disasters in recorded history (the other two being the North and South Towers), the debris of the building should have been meticulously examined. Many individuals and publications, such as the Fire Engineering Magazine, protested strongly, but in vain, against the rapid destruction of the evidence.

Witness statements

Craig Bartmer, a NYPD officer, states that he saw WTC 7 come down and heard a number of explosions in rapid succession. He is convinced that the skyscraper was brought down with explosives.

Barry Jennings, the deputy director of the Emergency Services Department of the New York City Housing Authority, has given a detailed statement of explosions in the evacuated WTC 7. As a result of a major explosion on the sixth floor, he and his colleague were trapped inside the building for about an hour and a half before the firemen were able to help them out. The colleague was Michael Hess, New York City’s corporation counsel, who confirmed to UPN 9 News that morning that an explosion in WTC 7 had trapped him and Jennings inside. Jennings emphasized that after the explosion, both of the twin towers were still standing, a statement that is supported by the time of Hess' interview in the morning. The men's statements, combined with the time of Hess' interview, place the explosion around 9:15–9:30, when the two were descending the stairs from the Mayor's Emergency Management Centre on the 23d floor, to which they had gone before the second plane struck the South Tower, only to find the center deserted.

Several rescue personnel have also come forward saying they were told that the building would be brought down by means of explosives. One such statement can be heard in this excerpt of Italian TV's documentary, in which one can also hear explosions from WTC 7: in one scene, a loud explosion is shown startling first responders, while a police officer says "the building is about to blow up".

Final Words

Was WTC 7 destroyed as a result of controlled demolition? Everyone can draw their own conclusions from the way in which the building was destroyed and the temperatures produced in the destruction.

If and when the building was demolished, it must have been wired with explosives before September 11th. An operation of that magnitude could not have been accomplished during a couple of chaotic hours. This is why the official hypotheses have not touched on the most obvious explanation for the collapse of the skyscraper. It is revealing that the 9/11 Commission, which published its report in 2004, does not mention in a single sentence the destruction of the third skyscraper resulting from the terrorist attack in New York.

FEMA's work was continued by the National Institute of Standards and Technology, which finally published its report on WTC 7 on November 20, 2008. NIST claims that thermal expansion resulting from ordinary office fires (not diesel fires) initiated a catastrophic chain of events leading to the total collapse of the skyscraper. The draft report was greeted with widespread disbelief and criticism on the part of both the general public and many experts. A group of architects, engineers and scientists refute NIST's conclusions point by point. The creator of this blog also approached NIST with his own comments.

Perhaps NIST's report needs to be analyzed in the light of how Dr S. Shyam Sunder, NIST's lead WTC investigator, interviewed in New York Magazine, summed up the state of the investigation back in 2006:

NIST did have "some preliminary hypotheses" on 7 WTC, Dr. Sunder said. "We are studying the horizontal movement east to west, internal to the structure, on the fifth to seventh floors." Then Dr. Sunder paused. "But truthfully, I don't really know. We've had trouble getting a handle on Building No. 7."